The invention relates to a bone anchoring device which comprises a receiving part for receiving a rod, an anchoring element having a first end for insertion into the bone and a second end held in the receiving part, and a locking device to limit a pivoting movement of the anchoring element to pivoting around a single axis.
Document US 2006/0155277 A1 discloses an anchoring element, which comprises a retaining means for receiving a rod, the retaining means having a ring-shaped mount, a fastening element for anchoring the device in the vertebra, and a securing element which can be screwed into a thread of the retaining means in order to fix the fastening element with an angle relative to the retaining means.
The fastening element comprises a threaded shank and a bearing, the latter being provided to achieve a pivotal motion when being supported by a separate intermediate element, which can be inserted into the ring-shaped mount of the retaining means. More specifically, the bearing includes spherical surfaces which engage with spherical counterparts of the intermediate element. The bearing further has two flat guiding surfaces formed on opposite sides thereof, which engage with respectively flat counter surfaces of the intermediate element.
The fastening element may perform a rotation movement around one single axis with respect to the intermediate element. However, the intermediate element can freely rotate within the mount around a longitudinal axis of the retaining means. Hence, a polyaxial adjustment of the fastening element relative to the retaining means is possible.
The final fixation is achieved by screwing the securing element into the thread of the retaining means after the rod is inserted. As a result thereof, pressure is exerted onto the rod, which transmits this pressure further to the intermediate element which then frictionally clamps the bearing. Thus, upon fixation by the securing element, the degree of free movement is reduced from polyaxial to fully rigid at the same time.
For certain applications it is desirable to have a bone anchoring device which allows an adjustment of the angle between the bone anchoring element and the receiving part in one single plane.
Based on the foregoing, there is a need for a bone anchoring device, which simplifies handling of an anchoring device and increases its stability against external forces once the device has been fixated using a fixation element.